Abrasive and process of manufacturing the same



coarse pieces.

of a given size were mixed together and used PatentedMar. 13, 1934 ABRASIVE PROCESS OF MANUFAC- TUBING THE SAllIE Uriah M. Binkley, Evanston,

Carborundum Company, Niagara Falls,

111., assignor to The N. Y.,

a corporation of Pennsylvania No Drawing. Application July 27, 1932, Serial No. 625,197

3 Claims. (01. 51-279) This invention relates to an improvement in abrasives and the process of manufacturing the same.

This application is a continuation in part of my co-pending application, United States Serial No. 523,649, filed March 18, 1931.

I have discovered a new method of preparing granular abrasives, particularly those made from electrically fused materials which I find increases the sharpness and cutting power of grain made according to my process in comparison with similar properties of grain as ordinarily produced.

The effectiveness of abrasive grains in grinding depends upon the inherent hardness and toughness of the material of which the grain is composed, upon the'type of fracture characteristic ofthe material, and upon the shape and angularity of the individual particles. Of these factors my invention relates particularly to the shape and angularity of the particles.

In the early development of abrasives, ordinary ore dressing practices were adopted with little or no change. Lumps of material were reduced to size by sledging or otherwise until they could be put through a jaw or roll crusher and the crushed grain then either put through additional crushers of a similar type or circulated again through the original crusher in order to reduce the size of Finally all the crushed particles without regard to the crushing history of any particular group of particles.

It was later found that the properties of abrasives made from the same basic material could be improved for some purposes by subjecting the crushed particles to a so-called mulling action in which the corners of the grain were rounded off and any splintery particles broken into pieces having roughly equal dimensions in all directions. This mulling action was secured, for example, by running the grain through a tube or ball mill operated at a speed so low as to prevent cascading of the crushing media so that the process became one of attrition rather than one in which the abrasive particles were broken along relatively large sections.

It was found that such mulled grains did not scratch the work as much and stood up longer in service in abrading relatively tough metals. It was found, however, that in grinding substances which were not tough enough to break the abrasive particles readily, the cutting action was more rapid with particles which had not been mulled. In wood-Working for instance, thewood fibres do not ofier as great a resistance to the cutting action of, abrasive grains as is experienced in metal grinding. Wood is fibrous in character and relatively soft, so that the sharp abrasive particles retain their angularity and cutting edges longer in wood-working than in metal grinding.

Mohs scale in each case, as compared with 9.1

on the Mohs scale for the fused alumina), and also have other points of inferiority. Silica has generally been used in the form of particles which, while angular, lack the sharp edges characteristic of the fracture found with fused alumina. Garnet, of the type suitable for abrasives, while angular, is likewise not so sharp as fused alumina;

is relatively friable; and is diflicult to secure in a high degree of uniformity. Siliconcarbide, while very hard, is relatively brittle. Abrasives made from electrically fused products such as alumina, however, are much tougher than the foregoing and can be secured in almost unlimited quantities in a high degree of uniformity. When prepared by my improved method, they are superior in this respect, as well as in toughness and shape, to the so-called natural abrasives and to silicon carbide for certain purposes.

I have found that even in the ordinary process of crushing, as heretofore practiced and described above, there is a certain amount of mulling action when particles which are reducedalmost to the desired size are again subjected to a crushing action, particularly when the crusher is heavily fed. By selecting only those particles which have not been subjected to such secondary crushing of particles near their own size, I am able to secure an abrasive product possessing, to an outstanding degree, the properties of sharpness, angularity, and even splinteriness, which are the antithesis of the properties produced by mulling.

Fused alumina is produced by melting calcined bauxite, or other aluminous material, in an electric arc furnace.

The molten material is allowed to solidifyas a large lump which is reduced in size to pieces .which will pass a screen having wires set 3" apart.

This material is further reduced in size to pieces whose maximum dimensions lie between and It is after this stage that my improved process differs fromthose previously in use.

I take the material which is to A" and finer, and after screening out all material which will pass approximately a 6 mesh screen, I pass the coarser material through crushing rolls. From the material thus crushed, I again sift out the particles which will remain on a screen having 6 wires to the linear inch and pass this coarse material through an additional set of rolls, after which I again sift the coarse material out in a similar manner and again repeat the operations as many times as may be necessary until all the material passes the 6 mesh screen.

I then take the 6 mesh and finer material and sift it on a screen having'20 wires to the inch and discard all material remaining on the latter screen as unfit for my particular purpose. Such material may be recrushed and used for other purposes but I never admix the material produced by such secondary crushing with my sharp material after such crushing.

The material which is finer than approximately 20 grit I find to be characterized by extreme sharpness and angularity, which result in its being particularly well-fitted for abrasive Work on soft materials. I find, moreover, that when sifted into approximately uniform sizes, grain possessing this peculiar characteristic will not pack together as closely as other types of grain, and I have accordingly adopted this inability to pack closely as a criterion by which to judge the quality of a given lot of grain in respect to the sharpness of the particles comprising it.

I believe the angularity of the grain to be due to the fact that a large proportion of the particles are formed directly by disruption of much larger pieces in a single passa e through the rolls so that there is no tendency to round the edges or break off the projecting corners of the particles. Pieces thus formed from particles of at least three times their diameter by a single crushing operation, I designate as primary crushed. particles or grain.

Much of the material through 6 on 20 mesh is itself primary crushed grain but this grit range is too coarse for ordinary abrasive use on soft materials.

The ability of the. grain to pack is measured in the following manner: About 500 grams of the grain to be tested is placed in a cylindrical brass container 1%" inside diameter and 14" deep. A loosely fitting plunger weighing about 500 grams is then placed on top of this grain and the container and its contents then raised along its axis and allowed to drop freely about 3" onto a wooden base. This bumping is repeated 150 times after which it is found that further jolting produces little change in the volume of the grain contained in the cylinder. The apparent density of the grain (grams per 0. c.) is then calculated from the weight of the grain and the volume which it occupies in the cylinder.

To illustrate the difference between my new product and those previously produced, I submit the following table showing the arbitrarily established limits which I use to determine the proper classification of various lots of grain. The ranges covered by each class are of approximately equal breadth; e. g., the density of 20 grit material of the new type may go as low as 2.08 at times.

A pparent density ranges Grit sizes Mulled Regular New Over 2. 22 2. 15-2. 22 Under 2.15 Over 2. 20 2. 14-2. 20 Under 2. 14 Over 2. 19 212-2. 20 Under 2. 12 Over 2. l2 2. 06-2. 12 Under 2.06 Over 2. l0 2. 00-2. 10 Under 2.00

large differences in performance of the-lots of abrasive material produced from an identical lot of fused material.

While I have selected fused alumina as an example in the description given above, it will be understood that this is for illustrative purposes and that my invention is also applicable to other abrasive products, particularly those synthetic abrasives which are made by fusion in the electric furnace and in which the hardness is materially greater than that of garnet or quartz.

Having thus described the invention. what I claim as new 1. In the manufacture of splintery abrasive grain, the steps which comprise subjecting lumps of abrasive material having average dimensions greater than a sixth of an inch to a primary crushing operation, and segregating the particles having average dimensions less than a twentieth of an inch for use in grinding operations requiring abrasive grain of pronounced angularity.

2. Granular abrasive material characterized by qualities substantially those of granules crushed from pieces of fused alumina at least three times their resultant diameter in a single crushing.

3. In the manufacture of fused alumina abrasive material, the process which comprises s electing only those grains which have been di-E,

rectly crushed in a single operation from pieces having at least three times their diameter.

URIAH M. BINKLEY. 

